A light-emitting diode (LED) and a laser diode (LD) which emit short-wave visible light from reddish orange band to bluish purple band, near ultraviolet, and ultraviolet band are fabricated using Al.sub.x Ga.sub.y In.sub.z N (0.ltoreq.X, Y, Z.ltoreq.1, X+Y+Z=1) crystal materials classified as group III nitride semiconductors (see, for example, Japanese Examined Patent Publication No. S55-3834). Also, group-III nitride semiconductors such as Al.sub.x Ga.sub.y In.sub.z N.sub.Q M.sub.1-Q (0.ltoreq.L, Y, Z.ltoreq.1, X+Y+Z=1; where M represents a group-V element other than nitrogen, and 0&lt;Q&lt;1) containing group-V elements such as phosphorus (P) and arsenic (As) in addition to nitrogen (N) have been used for fabricating group-III nitride semiconductor light-emitting devices (see Japanese Unexamined Patent Publications No. H4-192585, H4-192586, H10-84163 and EP No. 0496030A2).
In these group-III nitride semiconductor light-emitting devices, a light-emitting portion has heretofore been formed usually by a pn-junction type double hetero (DH) structure which conveniently emits a high intensity luminescent light (see Japanese Unexamined Patent Publication No. H6-260283). N and p-type cladding layers forming the light-emitting portion of the DH structure by interposing a light-emitting layer therebetween have heretofore been made usually of Al.sub.x Ga.sub.y N (0.ltoreq.X, Y.ltoreq.1, X+Y=1) (Jpn. J. Appl. Phys., Vol. 32, 1993, pp. L8-L11). The light-emitting layer is practically made of n-type Ga.sub.y In.sub.z N (0&lt;Y, Z&lt;1, Y+Z=1) (see Japanese Examined Patent Publication No. S55-3834). This is because a band gap convenient for obtaining a light with a wavelength from near ultraviolet region to short-wave visible light region, that is, about 360 nm to about 560 nm, is given by adjusting the indium composition ratio (=Z).
For example, Ga.sub.0.94 In.sub.0.06 N which has an indium composition ratio (=Z) of 0.06 is used for blue band LEDs composed of a single group-III nitride semiconductor light-emitting layer (see J. Vac. Sci. Technol. A, 13(3), 1995, pp. 705-710). An example in which Ga.sub.0.55 In.sub.0.45 N adopting a further increased indium composition ratio of 0.45 is used as a well layer is known (see Jpn. J. Appl. Phys., 34 (Part 2), No. 10B, 1995, pp. L1332-L1335).
A conventional example in which the light-emitting layer is formed by a single quantum well (SQW) structure or a multi quantum well (MQW) structure is known (see Mat. Res. Soc. Symp. Proc., Vol. 449, 1997, pp. 1203-1208). This is because if the quantum well structure is adopted, light-emitting spectrum can be narrowed, resulting in an excellent monochromatic light emission. An example in which a well layer provided in the SQW or MQW structure, which forms the light-emitting portion of a visible light-emitting device, is also made of Ga.sub.y In.sub.z N (0&lt;Y, Z&lt;1, Y+Z=1) is also known (see Jpn. J. Appl. Phys., 35 (Part 2), No. 1B, 1996, pp. L74-L76).
As a matter of course, a barrier layer located at a position facing the well layer is made of a group-III nitride semiconductor material showing a larger band gap than that of the constituent material of the well layer. In the conventional example, the barrier layer is usually made of Al.sub.x Ga.sub.y N (0.ltoreq.X, Y&lt;1, X+Y=1) (see Japanese Unexamined Patent Publication No. H10-163571). It is usual that, regardless of the SQW and MQW structures, light with a wavelength shorter than that corresponding to the band gap of Ga.sub.y In.sub.z N forming the well layer arises from the light-emitting layer of the quantum well structure adopting a conventional rectangular potential structure type owing to a quantum level created in the well layer.
It is also known in the art that the light-emitting layer can be composed of a layer having a strain, i.e., a strained layer (see Japanese Unexamined Patent Publication No. H7-297476). In this prior art, In.sub.0.2 Ga.sub.0.8 N having a thickness of 7 nm is used for a well layer that is a light-emitting layer. On the other hand, a strained-layer super lattice (SLS) structure constructed by stacking the strained layer is mainly utilized as constituent components other than the light-emitting portion. For example, the SLS structure made of Al.sub.y Ga.sub.1-x-y In.sub.x N (0.ltoreq.x, y.ltoreq.1, 0.ltoreq.x+y&lt;1) is utilized as a dislocation-reduction-layer for preventing dislocations in a buffer layer from propagating to an active (light-emitting) layer of a DH structure light-emitting portion (see Japanese Unexamined Patent Publication No. H8-264833). Moreover, prior art in which a SLS structure made of Al.sub.d Ga.sub.1-c-d In.sub.c N (0.ltoreq.c, d.ltoreq.1, 0.ltoreq.c+d&lt;1) and Al.sub.y Ga.sub.1-y In.sub.x N (0.ltoreq.x, y.ltoreq.1, 0.ltoreq.x+y&lt;1) is located below the DH structure light-emitting portion is known in the art similarly to the prior art described above (see Japanese Unexamined Patent Publication No. H6-152072). Besides these, an example is known, in which a buffer layer is formed in an SLS structure made of AlN and GaN (see Japanese Unexamined Patent Publication No. H3-203388).
As described above, the conventional light-emitting portion is composed of the single layer, the quantum well structure, or the structure regarded as the quantum well structure. The light-emitting layer composed of the single layer is one composed of a single group-III nitride semiconductor layer numerically, not compositionally. In order to obtain a single light-emitting layer (well layer) made of Ga.sub.y In.sub.z N that emits light with a longer wavelength, it is necessary to form the light-emitting layer from a Ga.sub.y In.sub.z N layer having a large indium composition ratio (=Z). As a matter of course, with reference to the quantum well structure using the conventional rectangular potential structure, a well layer must be composed of a Ga.sub.y In.sub.z N layer having an even larger indium composition ratio compared to a light-emitting layer composed of a single layer. This is because the transition energy between the carriers increases owing to a quantum level created in the rectangular potential well layer.
On the other hand, from the viewpoint of growth technology for Ga.sub.y In.sub.z N forming the light-emitting layer and the well layer, it is necessary to lower a growth temperature to obtain the Ga.sub.y In.sub.z N having a large indium composition ratio (=Z). However, it has been reported that a Ga.sub.y In.sub.z N grown at a low temperature near 500.degree. C. shows poor crystallinity (see The Journal of the Institute of Electronics, Information and Communication Engineer, Vol. 76, No. 9 (September, 1993), pp. 913-917). The crystallinity of the group-III nitride semiconductor crystal layer forming the light-emitting layer appears as a level of a light-emitting intensity. In other words, the use of the Ga.sub.y In.sub.z N crystal layer with poor crystallinity is disadvantageous for obtaining a nitride semiconductor light-emitting device which emits a high intensity light.
If a light-emitting layer or a well layer capable of emitting visible light with a comparatively long wavelength can be constructed using a Ga.sub.y In.sub.z N crystal layer grown at a high temperature near 800.degree. C. (see J. Insti. Electron. Infor. Communi. Eng.), which has excellent crystallinity because of a low indium composition ratio, a group-III nitride semiconductor light-emitting device emitting high intensity light can be advantageously acquired. However, since its band gap at room temperature is increased with a decrease in the indium composition ratio (see Japanese Examined Patent Publication No. S55-3834), disadvantage that such Ga.sub.y In.sub.z N crystal layer is hardly used as a suitable constituent material for forming a light-emitting layer which emits light with a wavelength of a bluish green or a green bands occurs. Particularly, a rectangular potential well layer formed by the Ga.sub.y In.sub.z N crystal layer having the such comparatively low indium composition ratio is more disadvantageous to radiate a short-wavelength visible light with a wavelength of, for example, a green band.
Even when the Ga.sub.y In.sub.z N crystal layer having a low indium composition ratio (=Z), and excellent crystallinity is used as a constituent material of the single light-emitting layer or the potential well, it is possible to form a group-III nitride semiconductor light-emitting device emitting a high intensity luminescent light, provided that a light-emitting layer capable of readily emitting visible light with a longer wavelength can be developed.
Furthermore, if technical procedure for stably improving the crystallinity of the Ga.sub.y In.sub.z N crystal layer forming the light-emitting layer or the well layer consciously can be done, as a matter of course this makes it quite advantageous to obtain a group-III nitride semiconductor light-emitting device to emit a high intensity light in a more stable manner.